A mass-produced part is commonly manufactured to a specification which defines a master model of the part using specific values of various physical characteristics such as geometric dimensions, characteristics of the part material, etc. A part specification also recognizes that physical characteristics of actual production parts may differ from those of the master model defined in the specification. Some part-to-part differences may be acceptable in a particular mass-production manufacturing process, but others are not. Consequently a specification for a part which is mass-produced in a particular manufacturing process may contain allowable tolerances for various physical characteristics of the part.
A plastic part is first manufactured by a molding process, such as blow molding, and is then further processed by various operations which for example, may include boring, drilling, and welding. Part-to-part variations in molded plastic parts may apply to almost any particular physical characteristic, but may be generally said to involve geometric differences, such as wall thickness and flatness for example, which can be caused by variations of melt flow rate, humidity of raw material, ambient temperature, etc. Those factors can cause significant part-to-part variation in finished parts when compared to the master model.
A common way of defining a particular dimension of a mass-produced part is by specifying an acceptable range for the particular dimension, such as a length of 100.0 mm.+/−1.5 mm., which defines an acceptable range in length from, and including, 98.5 mm. to, and including, 101.5 mm. Stated another way, the dimension has a nominal length of 100.0 mm. and a tolerance on that length of +/−1.5 mm. Any length within that range is said to be within tolerance. Any length not within that range is said to be out-of-tolerance.
The nature of a particular material or materials of a mass-produced part and the nature of the industrial machinery which performs operations on those parts may have significant effect on dimensional tolerances for the part. For example, mass-produced machined steel parts can be fabricated with a greater degree of precision than can mass-produced parts which are fabricated by certain molding processes and whose material is less rigid than steel, such as blow molding of plastic parts. Consequently when further processing operations, such as drilling and boring for example, are performed on those two types of parts, modern machine tools can perform those operations on steel parts with precision which allows a specification for such a part to have very small tolerances while comparable operations on mass-produced molded parts whose material is less rigid than steel do not allow a specification for such a part to have the small tolerances which apply to the steel part.
When the process for manufacturing a mass-produced part involves using a machine to perform one or more of operations on a part, the part is placed in a particular three dimensional relationship to the machine. Because of part-to-part dimensional differences, the location where a particular operation is performed on one part may differ from the location where the same operation is performed on another part. As long as each of those two parts is within tolerance before the operation is performed, and the path of motion of an element of a machine which performs the operation is invariant from part-to-part, dimensionally correct parts will be manufactured.
A machine which comprises an industrial robot can position an element which performs an operation on a part during a manufacturing process with a high degree of accuracy and repeatability. Consequently, it is essentially part-to-part variation, and not the machine, which is the predominant cause of a finished manufactured part being out-of-tolerance.
Certain parts which are fabricated by a molding process such as blow molding may be subject to localized variations in thickness, to variations in locations of certain features such as holes, and to variations in shapes of certain features. For example, a zone which should ideally be flat may be bumpy or it may be tipped out of an imaginary plane which it should occupy.
When a machine having an industrial robot for positioning a heated welding plate which is to perform a welding operation at a particular location on a molded part which is subject to significant part-to-part variation, the inability of the machine to self-adjust itself to compensate for part-to-part variation at the location on the part where the operation is to be performed, the thickness of the part at that location, and the shape of the part at that location, may produce an unacceptable number of out-of-tolerance parts.